1. Field of the Invention
The present invention relates to the reference voltage source and current source circuits, and more particularly, to the voltage source and current source circuits using a source follower.
2. Description of the Related Art
The reference voltage source and current source circuits are widely utilized in the analog circuit for providing a DC parameter that has lower correlation to the fabricating process parameter. FIG. 1 schematically shows the conventional voltage source and current source circuits. Referring to FIG. 1, the conventional voltage source and current source circuits comprise an amplifier 101, a current mirror circuit 102, an NMOS transistor MN1, and two resistors R0˜R1. Wherein, the negative input terminal of the amplifier 101 is electrically coupled to the source of the transistor MN1 and is grounded through the resistor R0. The current mirror circuit 102 is electrically coupled to the drain of the transistor MN1 and sequentially outputs a reference voltage VREF and a reference current IREF.
The current mirror circuit 102 comprises three PMOS transistors MP1˜MP3. The sources of the PMOS transistors MP1˜MP3 are electrically coupled to a DC bias VDD, and the gates of the PMOS transistors MP1˜MP3 jointly coupled with each other are electrically coupled to the drain of the PMOS transistor MP1.
Referring to FIG. 1, after the voltage VBG is amplified by the amplifier 101, a voltage signal is generated on the source of the transistor MN1. Meanwhile, the voltage on the node A, i.e. the output voltage of the amplifier 101, can be represented as VBG+VTN, where VTN is a threshold voltage of the transistor. For providing high portability to the modern electronic products, the analog circuit is usually operated under a lower DC bias, such that the purpose of lower power consumption is achieved. However, when the conventional voltage source and current source circuits are operated under a lower voltage, the output voltage of the amplifier 101 is easily deviated from the ideal output voltage level. For example, if the DC bias VDD is 2.5V and VBG+VT≈2.2V, meanwhile the output voltage of the amplifier 101 is far deviated from the ideal output point VDD/2 (1.25V). Accordingly, the signal source of the conventional voltage source and current source circuits are not stable, which further impacts the voltage gain and generate noises.
Moreover, under the low voltage operation, the output voltage of the amplifier 101 may be too close to the DC bias VDD, which constraints the selection of the configurations for fulfilling the requirements of the full-swing output voltage.